Wind turbines include a rotor that supports a number of blades extending radially therefrom for capturing the kinetic energy of the wind and causing a rotational motion of a driving train coupled to an electric generator for producing electrical power.
The amount of energy produced by wind turbines is dependent on the rotor blade sweeping surface that receives the action from the wind and consequently increasing the length of the blades leads normally to an increase of the power output of the wind turbine.
However, the size of the blades for land-based wind farms is presently limited to some extent by transportation and infrastructure requirements. In particular, the size of bridges and tunnels limit the size of the blade maximum chord.
To solve the transportation problems posed particularly by lengthy blades the prior art teaches the division of the blade in two or more longitudinal sections provided with joining means, so that each section may be manufactured individually and all sections may be assembled at the wind turbine site. Examples of this prior art are the following.
DE 3 109 566 discloses a wind turbine blade subdivided into at least two longitudinal sections which are held together by an expanding bolt.
U.S. Pat. No. 4,389,182 discloses a wind turbine blade subdivided into several longitudinal sections that are interconnected by tensioning members such as steel cables extending through the blade sections.
EP 1 244 873 A1 discloses a wind turbine blade subdivided into longitudinal sections that are joined by means of a butt joint comprising a number of clips arranged along the joint, having the respective ends fixed to the sections to be joined, and bolts for fixing said clips.
WO 2005/100781, WO 2006/103307, WO 2007/051879 in the name of the applicant, disclose wind turbine blades subdivided into longitudinal sections having improved joining means.
There is also prior art teaching the division of the blade in several transversal sections in addition or independently to the division in longitudinal sections. Examples of this prior are the following.
EP 1 184 566 A1 discloses a wind turbine blade which is formed by assembling one, two or more longitudinal sections, each of which comprises a core formed by a longitudinal carbon-fibre tube on which a series of carbon fibre or fiberglass cross ribs are mounted and a cover formed by fiberglass or carbon-fibre joined to said ribs.
WO 01/46582 A2 discloses a wind turbine blade having a plurality of segmented elements attached to a load transmitting box spar and separated by elastic joins which enable the segments to move in relation to one another in order to minimise the tensile stress in the region of the blade in which the segments are located.
EP 1 965 074 in the name of the applicant discloses a wind turbine blade composed of two cap prefabricated panels and two web prefabricated panels placed side by side in a box shape and at least two shell longitudinal sections forming, respectively, the leading edge and the trailing edge of the corresponding blade section that are placed adjacently to a central spar section, the aerodynamic profile of the blade being defined by said cap panels and said shell panels.
The current trend in the wind industry to big rotor blades demands new rotor blades designs suitable for complying with the transportation requirements and with the quality manufacturing requirements involved by blades that can reach lengths of 100 m and maximum chords of 8 m.
The present invention is intended to satisfy said demand.